Apparatus for plasma-enhanced physical vapor deposition of copper with RF source power applied through the workpiece with a lighter-than-copper carrier gas

ABSTRACT

A method of performing physical vapor deposition of copper onto an integrated circuit in a vacuum chamber of a plasma reactor includes providing a copper target near a ceiling of the chamber, placing an integrated circuit wafer on a wafer support pedestal facing the target near a floor of the chamber, introducing a carrier gas into the vacuum chamber having an atomic weight substantially less than the atomic weight of copper, maintaining a target-sputtering plasma at the target to produce a stream comprising at least one of copper atoms and copper ions flowing from the target toward the wafer support pedestal for vapor deposition, maintaining a wafer-sputtering plasma near the wafer support pedestal by capacitively coupling plasma RF source power to the wafer-sputtering plasma, and accelerating copper ions of the wafer sputtering plasma in a direction normal to a surface of the wafer support pedestal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 11/052,011, filed Feb. 3, 2005 now U.S. Pat. No. 7,399,943 entitled “APPARATUS FOR METAL PLASMA VAPOR DEPOSITION AND RE-SPUTTER WITH SOURCE AND BIAS POWER FREQUENCIES APPLIED THROUGH THE WORKPIECE” by Karl M. Brown et, al. and assigned to the present assignee.

BACKGROUND OF THE INVENTION

With recent technological advances in integrated circuit design, there are now as many as six to ten interconnect layers of a multilayer interconnect structure overlying the semiconductor transistors. Insulator layers separate the successive conductor layers. The conductor interconnect layers can have completely different conductor patterns and are connected to one another and to the transistor layer at different locations through vias extending vertically through the insulator layers. It is the formation of the vias with which the present invention is concerned.

To reduce resistive power losses in the integrated circuit, the interconnect layers and the vias typically employ aluminum and, more recently, copper as the principal conductor. The insulator layers are silicon dioxide, although more recently dielectric materials other than silicon dioxide having a lower dielectric constant are increasingly being employed. Because copper tends to diffuse over time through the insulator layer to cause catastrophic short circuiting, a barrier layer that blocks copper diffusion is placed between the copper material and the dielectric material wherever the two materials interface in the integrated circuit. The barrier layer is typically formed of an underlying tantalum nitride or titanium nitride layer contacting the insulator layer, an overlying pure (or nearly pure) tantalum (or titanium) layer and, finally, a copper seed layer over the pure tantalum (or titanium) layer. If the conductor is to be copper, then tantalum is preferred. The copper conductor is deposited on the copper seed layer. Such a barrier layer prevents migration or diffusion of copper atoms into the dielectric material. The tantalum and tantalum nitride (or titanium and titanium nitride) layers are poor conductors compared to copper. The formation of the barrier layer and of the copper conductors is carried out by physical vapor deposition. Other deposition processes may be employed for the metal fill step (copper deposition) such as chemical vapor deposition, plasma enhanced chemical vapor deposition or electroplating.

A problem arises in forming the vias extending vertically between the horizontal interconnect layers, as follows. Each vertical via opening is formed by etching a hole through an overlying horizontal insulator (silicon dioxide) layer so as to expose a portion of the copper conductor in the underlying horizontal interconnect layer. It is this exposed portion to which connection is to be made through the via. A barrier layer must be deposited on the interior surfaces of the via before the copper conductor can be formed in the via, to prevent copper migration as explained above. This barrier layer, in covering all interior surfaces of the via, covers the exposed portion of the underlying copper conductor. Since the barrier layer is an inferior conductor, it must be selectively removed from the underlying copper conductor (in an etch process) without removing the remainder of the barrier layer from the other interior surfaces of the via. This removal step has required interruption of the physical vapor deposition process in order to place the wafer in an etch chamber where the step of selectively removing the barrier layer from the underlying copper surface is carried out. The wafer is then returned to a physical vapor deposition reactor for formation of the copper conductor(s).

The interruption represented by the selective removal of the barrier layer entails a higher production cost and consumes production time. In recent years, a dual purpose reactor was developed capable of both physical vapor deposition of the barrier layer and selective removal of the barrier layer after the barrier layer formation step, without removing the wafer from the reactor. As a result, great savings in production cost and production time have been realized. This was accomplished by providing in the physical vapor deposition chamber a separate coil near the wafer. After barrier layer formation, the coil is used to form an inductively coupled plasma which selectively sputters the barrier layer from horizontal surfaces (i.e., the floor formed by the underlying copper conductor). Such selective sputtering (hereinafter referred to as “re-sputtering”) is achieved by applying RF bias power to the wafer to achieve an ion velocity distribution that is predominantly vertical. While this dual-purpose reactor works extremely well, it does entail some additional expense. For example, since the barrier layer deposition step involves sputtering a metal target and therefore deposits metal over all interior surfaces of the reactor chamber, the re-sputtering coil must be located inside the chamber so that no metallized surfaces shield the coil or otherwise prevent inductive coupling of RF power from the re-sputtering coil to the plasma. In order to avoid process contamination, the re-sputtering coil is formed of pure tantalum, adding cost. The coil is subject to very large temperature fluctuations during its lifetime, and must be changed periodically. RF power must be coupled to the re-sputtering coil through the vacuum seal of the reactor chamber and through an environment that periodically is completely filled with metal vapor. Therefore, an RF feedthrough must be employed that can tolerate the metal deposition, and whose exterior surfaces are textured to avoid excessive accumulation of deposited materials and flaking, and that can tolerate wide temperature excursions over its lifetime.

Another well-known dual-purpose reactor employs an external inductive coil overlying a portion of the ceiling not blocked by the metal sputter target. One problem is that the metal vapor deposition process can coat the ceiling with metal and thereby block inductive coupling from the coil antenna. A more significant problem is that the RF plasma produced by the coil produces a high proportion of metal ions from the target, so that the wafer bias cannot be optimized for etch selectivity to horizontal surfaces without impacting (de-optimizing) the flux of metal ions/vapor from the target. Therefore, the metal deposition process and the re-sputter process must be performed at separate times.

It should be noted that although such dual purpose reactors are capable of performing both the Ta/TaN barrier layer deposition step and the re-sputtering step, a different reactor is typically employed to perform the subsequent copper deposition step. This is because a high flux of copper ions on the wafer is desired, and the PVD reactor must be specially configured in order for the sputtered copper atoms to self-ionize in a very dense plasma at the copper target. Specifically, a very high D.C. power level (40-56 kWatts) is applied to the copper target and a specially configured magnetron is employed for a more concentrated plasma at the target. Because of the high density of copper ions near the target, it is placed very high above the wafer (390 mm), which limits the copper deposition rate to an acceptable threshold (as well as providing some beneficial collimation of copper neutrals). Typically, however, are large share of the copper ions are deposited on shields in the chamber and otherwise lost while traveling over this long distance.

In addition to the requirement for a copper PVD chamber and a barrier PVD chamber, a third chamber, an etch chamber, must be employed to carry out a pre-deposition cleaning process, since the copper PVD chamber and the barrier PVD/re-sputter chamber are not suitable for clean/etch processes.

Another problem is the tendency of the tantalum and/or tantalum nitride material deposited during formation of the barrier layer to deposit with non-uniform thickness along the via walls, and in particular to accumulate faster near the top corners of the vertical walls and thereby exhibit some tendency toward pinch-off. This makes it necessary to restrict the process window in order to ameliorate such problems. This problem is solved to some extent when, upon completion of the barrier layer deposition process, the re-sputtering process is performed, because the re-sputtering process tends to remove tantalum or tantalum nitride from the tops and corners of the via walls faster than elsewhere, while transferring tantalum (or tantalum nitride) material removed from the horizontal surfaces (floors) of the vias onto the lower portions of the via sidewalls. Nevertheless, it would be beneficial to avoid altogether the initial non-uniform tantalum or tantalum nitride deposition problem, to eliminate any risk of pinch-off, permitting some liberalization of the process window.

It would also be beneficial to avoid the necessity of the internal re-sputtering coil provided at least some of its benefits could be realized in a simpler manner.

The present invention provides benefits at least approaching those afforded by the internal re-sputtering coil without the need for such a coil. The present invention furthermore provides a way of ameliorating or avoiding non-uniform deposition of the barrier layer, and a way of avoiding or minimizing deposition of the barrier layer on the exposed copper conductor surface forming the floor of the via during formation of the barrier layer.

SUMMARY OF THE INVENTION

A method of performing physical vapor deposition of copper onto an integrated circuit in a vacuum chamber of a plasma reactor includes providing a copper target near a ceiling of the chamber, placing an integrated circuit wafer on a wafer support pedestal facing the target near a floor of the chamber, introducing a carrier gas into the vacuum chamber having an atomic weight substantially less than the atomic weight of copper, maintaining a target-sputtering plasma at the target to produce a stream comprising at least one of copper atoms and copper ions flowing from the target toward the wafer support pedestal for vapor deposition, maintaining a wafer-sputtering plasma near the wafer support pedestal by capacitively coupling plasma RF source power to the wafer-sputtering plasma, and accelerating copper ions of the wafer sputtering plasma in a direction normal to a surface of the wafer support pedestal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away side view of a plasma reactor embodying the invention.

FIG. 2 is an enlarged cut-away view of a wafer support pedestal in accordance with one aspect.

FIG. 3 is an enlarged cut-away view of a wafer support pedestal in accordance with another aspect.

FIG. 4 illustrates the use of two different plasma bias power frequencies in the reactor of FIG. 1.

FIG. 5 is a graph illustrating the combination of the different ion energy distributions of the different bias power frequencies in the embodiment of FIG. 4.

FIGS. 6A through 6E are sequential cross-sectional views of a portion of an integrated circuit, in accordance with a process of the invention.

FIG. 7 is an enlarged cross-sectional view of a barrier layer formed in the process of FIGS. 6A through 6E.

FIGS. 8A through 8C illustrate the formation of a barrier layer in one embodiment of a process of the invention.

FIGS. 9A and 9B illustrate the formation of a barrier layer in the preferred embodiment of a process of the invention.

FIG. 10 is a block flow diagram of a preferred process of the invention.

FIGS. 11A, 11B and 11C are cross-sectional side views of an narrow opening through a dielectric layer, and depict deposition results in three respective modes of the reactor of FIG. 1, namely a conformal mode, a non-conformal mode and a punch-through mode, respectively.

FIG. 12 illustrates a plasma reactor in accordance with a first alternative embodiment.

FIG. 13 illustrates a plasma reactor in accordance with a second alternative embodiment.

FIG. 14 illustrates a plasma reactor in accordance with a third alternative embodiment.

FIG. 15 illustrates a plasma reactor in accordance with a fourth alternative embodiment.

FIG. 16 illustrates a plasma reactor in accordance with a fifth alternative embodiment.

FIG. 17 depicts a process in which the reactor of FIG. 1 performs a pre-deposition cleaning process, a dielectric barrier layer deposition process and a metal barrier layer deposition process on the same wafer.

FIGS. 18A and 18B illustrate one embodiment of an RF grounding conductive tab of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

A plasma reactor forms barrier layers (such as a tantalum/tantalum nitride film or titanium/titanium nitride film) for conductors (such as copper, for which the barrier should be tantalum/tantalum nitride) in trenches or through vias between successive interconnection layers of an integrated circuit. The plasma reactor is capable of both physical vapor deposition and of highly selective re-sputtering to remove barrier material from the exposed horizontal surfaces of the underlying conductor constituting the floor of the via. Significantly, the reactor accomplishes all this without an internal coil that had previously been required for a fully and precisely controllable re-sputtering step. Instead, a plasma is formed near the wafer to perform the re-sputtering step. For this purpose a process gas such as argon may be introduced and source power is applied to the wafer at an RF frequency effective for capacitively coupling energy to kinetic electrons to excite argon plasma ions near the wafer. The term “source power” as employed in this specification refers to RF power suitable for maintain an RF-coupled plasma by coupling power to kinetic electrons in the plasma. This is to be distinguished from use of the term “source” when referring to the D.C. excited metal sputtering target, which is the “source” of the metal atoms or ions in a metal deposition process. Typically, the source power frequency is a VHF frequency because of the low mass-to-charge ratio of electrons. The ions of the VHF-coupled plasma formed near the wafer are employed in the re-sputtering step. The selectivity of the re-sputtering step for horizontal surfaces is established by applying bias power to the wafer at an RF frequency effective for coupling energy to the ions (e.g., argon ions), which is typically an HF or LF frequency because of the high mass-to-charge ratio of the ions. This constricts the ion velocity distribution across the plasma sheath at the wafer surface to a small directional range about the chamber axis, typically a vertical direction, making the re-sputtering step highly selective for surfaces perpendicular to the chamber axis, typically horizontal surfaces. A significant feature is that the bias power controls the selectivity of the ion re-sputter or etch step without affecting the flux of metal atoms from the target toward the wafer. This is facilitated by a low power (2-5 kW) D.C. discharge plasma at the target surface which produces primarily neutral metal particles or atoms from the target that are unaffected by the bias power applied to the wafer. Thus, two different plasmas are employed, one being a D.C. discharge plasma at the target and the other being an RF (VHF) plasma at the wafer. Therefore, the target sputtering may be optimized without affecting the re-sputtering plasma at the wafer, while the bias voltage may be optimized without affecting the target sputtering. This feature is not possible in conventional ion physical vapor deposition reactors that employ RF coupled plasmas near the target to produce metal ions for the physical vapor deposition process.

An advantageous mode is provided that is unique to the present invention in which the physical vapor deposition and the re-sputtering can be performed simultaneously, and the selectivity of the re-sputter or etch process is adjusted without affecting the flux of metal atoms from the target.

The re-sputtering step tends to compensate for non-uniform deposition of the barrier layer material. Therefore, in one embodiment of the invention, the re-sputter step is performed simultaneously with the barrier deposition step. This is possible because a preferred embodiment of the invention produces two somewhat independent plasmas, namely a D.C. discharge plasma near the ceiling or target that is confined by a magnetron above the target and an RF plasma near the wafer surface to perform the re-sputtering. Thus, the plasma near the ceiling is optimized for sputtering the target while (simultaneously) the plasma at the wafer is optimized for re-sputtering and selectively etching the floor of each via. One advantage is that prominent non-uniformities in the deposited barrier layer film are reduced or are never formed, thereby reducing the risk of pinch-off or other problems in the vias. Another advantage of this embodiment is that throughout the entire barrier deposition/re-sputter process, accumulation of barrier material can be entirely avoided on the exposed horizontal surface of the underlying conductor forming the floor of the via. This is accomplished by adjusting the barrier material deposition rate (controlled largely by the tantalum target D.C. sputter voltage) relative to the re-sputter rate (controlled largely by the VHF plasma source power applied to the wafer.)

The invention can afford certain advantages depending upon it application. For example, a low power (2-5 kW) D.C. power level is employed to sputter the metal target for all deposition process, including copper, tantalum and titanium, because metal neutrals are desired. Therefore, the target height above the wafer and the magnetron design are the same in all these processes, so that the same chamber can perform any or all of them. The target height above the wafer can be relatively low, or between about 225 mm and 290 mm, for example. In addition, the VHF source power applicator and the HF bias power applicator can be used without a target to excite a plasma (such as argon plasma) to perform a pre-deposition clean process prior to each deposition process. Such a pre-clean process can be repeated prior to each and all of the deposition steps, including the barrier metal (tantalum) deposition step, the barrier dielectric (tantalum nitride) barrier deposition step, the copper seed layer deposition step and the copper conductor deposition step.

Reactor Apparatus:

Referring to FIG. 1, a reactor of a first embodiment of the invention includes a vacuum chamber defined by a cylindrical side wall 10, a disk-shaped ceiling 12, and a wafer support pedestal 14 for supporting a semiconductor wafer 16 to be processed. A target 18 of a metal (e.g., tantalum) to be deposited on the wafer 16 is mounted on the ceiling 12. A process kit consisting of a cylindrical shield 20 surrounds the wafer 16 and the target 18. A magnetron 22 of the conventional type overlies the target 18 on the external side of the ceiling 12. A high voltage D.C. source 24 is coupled to the target 18. A process gas injector 26 furnished process gas from a supply 28 into the interior of the chamber. A vacuum pump 30 maintains a desired sub-atmospheric pressure in the vacuum chamber.

The wafer pedestal 14 is coupled through an impedance match network 34 to a VHF plasma source power generator 36 and to an HF or LF plasma bias power generator 38. The high voltage D.C. source maintains an upper plasma 40 near the target 18. The VHF plasma source power generator 36 maintains a lower plasma 42 at or near the surface of the wafer 16. The two plasmas 40, 42 may be maintained simultaneously or may be produced at different times. The upper plasma 40 is a D.C. discharge plasma that enables sputtering of the target 18 to produce mainly neutral metal atoms from the target 18 that deposit on the wafer, with some metal ions from the target 18. The lower plasma 42 is a capacitively coupled RF plasma that promotes selective etching of horizontal surface on the wafer 16. The two plasmas 40, 42 may be controlled independently, for separate control of the metal deposition process and the re-sputter process. The LF bias power applied to the wafer determines the selectivity of the re-sputter/etch process for horizontal surfaces. Plasma uniformity, particularly uniformity of the plasma 42 nearest the wafer, is controlled by an electromagnetic coil 43 wrapped around the cylindrical sidewall of the reactor chamber and supplied with D.C. current by a current source controller 45.

Coupling of the VHF source power and HF or LF bias power to the wafer is illustrated in FIG. 2. The wafer support pedestal 14 can be an electrostatic chuck (ESC) for electrostatically holding the wafer 16 in place. In this case, the ESC or pedestal consists of an insulating layer 50 mounted on a conductive base 51 and an electrode 52 such as a conductive mesh buried inside the insulating layer 50. The pedestal structure can extend downwardly in a coaxial arrangement consisting of a center conductor 54 connected to the electrode 52, an insulating intermediate layer 56 and an outer conductor 58 connected to the conductive base 51. The conductive base can be coupled to the bottom of the cylindrical shield 20 by conductive tabs 60 to provide a more continuous ground reference. The center conductor 54 is coupled to the RF power sources while the outer conductor 58 is grounded. The impedance match network 34 consists of a conventional impedance match circuit 64 coupled to the RF (VHF) plasma source power generator 36 and a conventional impedance match circuit 68 coupled to the RF (HF or LF) plasma bias power generator 38. The output of the impedance match circuit 64 is connected through a high pass filter 65 to the wafer pedestal center conductor 54, while the output of the impedance match circuit 68 is connected through a low pass filter 69 to the wafer pedestal center conductor 54. In addition, a D.C. chuck voltage source 74 is connected to the wafer pedestal center conductor 54 and is isolated from RF power by an isolation capacitor 76. The high pass filter 65 has a sufficiently high cut-off frequency to prevent HF or LF current from the RF generator 38 from reaching the VHF match circuit 64, while the low pass filter has a sufficiently low cut-off frequency to prevent VHF current from the RF generator 36 from reaching the HF (or LF) match circuit 68.

FIG. 3 illustrates another embodiment of the wafer support pedestal 14 in which the electrode 52 contacts the wafer, and there is no electrostatic chucking of the wafer. In this case, since the electrode 52 is potentially exposed to the plasma, the electrode 52 can be formed of the material to be deposited on the wafer, such as tantalum.

FIG. 4 illustrates an embodiment in which the bias power consists of both HF and LF frequencies (e.g., 13.56 MHz and 2 MHz, respectively, for example). For this purpose, there are two bias power RF generators, namely an HF bias power generator 38 a and an LF bias power generator 38 b, the generators 38 a, 38 b being coupled through respective match circuits 68 a, 68 b to the wafer pedestal center conductor 54. The VHF source power generator 36 is coupled through its impedance match circuit 64 and through the high pass filter 65 to the wafer pedestal center conductor 54. One advantage of this embodiment is that the lower ion energy distribution of the HF bias and the higher ion energy distribution of the LF bias (both shown in FIG. 5) can be combined to produce an ion energy distribution that peaks between the peaks of the LF and HF ion energy distributions. This peak can be shifted up or down in energy by adjusting the relative power levels of the LF and HF power generators 38 a, 38 b.

The deposition rate of the target material (e.g., tantalum) is mainly determined by the power delivered by the D.C. voltage source to the target. The selectivity of the etch/re-sputter process (for horizontal surfaces) is determined by the bias power, while the rate of the etch/re-sputter process is mainly determined by the source power level. Therefore, there are three parameters that can be controlled independently of one another, namely metal deposition rate, etch selectivity for horizontal surfaces and etch rate. Since all these can be controlled independently of one another, the metal deposition and etch/re-sputter process may be performed simultaneously, if desired.

PVD/RE-Sputter Method:

The reactor of FIG. 1 is particularly useful in the formation of metal contacts between successive interconnection layers of an integrated circuit. Typically, an integrated circuit includes an active semiconductor layer having thousands of transistors and many insulated interconnection layers stacked over the active semiconductor layer and providing complex interconnection circuits among the transistors. The connections between interconnection layers are formed through vias or vertical holes between the interconnection layers by filling the holes with a metal such as copper. In order to prevent failure by short circuiting due to diffusion of copper through insulating material, a barrier layer of tantalum and tantalum nitride is placed between the copper and the insulating material. It is in the deposition of the barrier layer within the via that the reactor of FIG. 1 provides great advantage.

FIG. 6A is an enlarged cross-sectional side view of a partially fabricated portion of an interconnection layer 100 in a stack of many interconnection layers overlying an active semiconductor layer (now shown). FIG. 6B is the corresponding top view. The interconnection layer 100 includes, among many conductors, a pair of parallel copper conductors 102, 104 embedded in an insulator (silicon dioxide) layer 106. Each copper conductor 102, 104 is separated from the dielectric layer 106 by a barrier layer 108 that prevents diffusion of copper atoms into the dielectric layer 106. Preferably, the barrier layer 108 includes a tantalum nitride layer contacting the insulator layer 106 and covered by a tantalum layer and a copper seed layer. The structure is best shown in the enlarged view of FIG. 7, which shows a tantalum nitride layer 110 covering the insulator layer 106, a tantalum metal layer 112 overlying the tantalum nitride layer 110 and a copper seed layer 114 overlying the tantalum metal layer 112. A copper conductor, such as the copper conductor 102, overlies the copper seed layer 114. The tantalum metal layer 112 establishes a high quality bond with the tantalum nitride layer 110, while the copper seed layer establishes a high quality bond with the tantalum metal layer 112 on one side and the copper conductor 102 on the opposite side.

Before the next interconnection layer is formed on the top surface of the insulator layer 106, a vertical opening or via 120 is opened through the insulator layer 106, typically by etch processes (FIGS. 6A and 6B). The via 120 consists of a large opening 122 extending only partly through the insulator layer 106 and a pair of smaller openings 124 extending down to the respective copper conductors 102, 104. Typically, the etching process that forms the two smaller openings 124 is carried out sufficiently to remove the exposed portion of the barrier layer 108 overlying each conductor 102, 104 (FIG. 6A).

The via 120 forms a vertical conductor when later filled with copper. However, before copper is deposited into the via 120, a barrier layer 130 is deposited onto all surfaces in the via 120 and onto the top surface of the insulator layer 106 as shown in FIG. 6C. The barrier layer 130 has the same structure as that described above with reference to FIG. 7, including a tantalum nitride layer 110, a metal tantalum layer 112 and a copper seed layer 114. In one embodiment of the invention, each sub-layer 110, 112, 114 of the barrier layer 120 is deposited in a separate step by providing the appropriate material for the metal target 18 (tantalum for the sub-layers 110, 112 and copper for the sub-layer 114). The target 18 is sputtered by introducing a process gas which is ionized in the vicinity of the target by the large D.C. sputtering voltage from the sputter voltage source 24. In order to deposit the tantalum nitride sub-layer 110, nitrogen is employed as the process gas, and the tantalum atoms combine with nitrogen atoms as they impact the wafer to form a tantalum nitride film. When depositing the metal tantalum layer 112 and, later, when depositing the copper seed layer 114, the process gas is an inert or non-reactive gas, such as Argon. Thus, three deposition steps are carried out. First, using a tantalum sputtering target and nitrogen process gas, tantalum nitride is deposited. Second, using a tantalum target and argon process gas, metallic tantalum is deposited. Third, using a copper target and argon gas, a copper seed layer is deposited.

In one embodiment of the invention, no plasma RF (VHF) source power is applied to the wafer support pedestal 14, although a modest level of plasma RF (HF or LF) bias power may be applied. In this way, metal is sputtered from the target 18 and deposited onto the wafer 16. As a result, the barrier layer 130 is formed by carrying out the three deposition steps for the successive sub-layers 110, 112, 114. The barrier layer 130 covers all exposed surfaces including the portions of the copper conductors 102, 104 exposed by the small openings 124, as shown in FIG. 6C.

After each of the three deposition steps have been completed, an etch/re-sputter step is carried out (FIG. 6D) in which the VHF plasma source power generator 36 and the HF (or LF) plasma bias power generator 38 deliver power to the wafer support pedestal 14. This produces a plasma near the wafer that furnishes ions guided to the horizontal surfaces by the applied plasma bias power. These ions sputter the horizontal surfaces to remove films deposited thereon, such as the portion of the barrier layer 130 at the bottom of the small openings 124. Within the small openings 124, the close proximity of the vertical walls (or small diameter of the openings 124) promotes the re-deposition of atoms sputtered from the floor 124 a of each opening 124 onto the vertical side wall. This uncovers the copper conductors, as desired, as illustrated in FIG. 6D. In other areas where there is no vertical wall, such as the vast area of the top surface of the insulator layer 106, the sputtered atoms re-deposit onto the horizontal surface, so that there is no net loss.

In a preferred embodiment, each of the three deposition process steps (corresponding to the three layers 110, 112, 114) is performed simultaneously with the etch and re-sputter process step. In this preferred embodiment, the sputter voltage source 24 delivers power to the target 18, while simultaneously the VHF plasma source power generator 36 and the HF (or LF) plasma bias power generator 38 deliver power to the wafer support pedestal 14. This produces a plasma near the wafer that furnishes ions guided to the horizontal surfaces by the applied plasma bias power while atoms from the target 18 are being deposited. Deposition of atoms from the target 18 on the horizontal surfaces such as the exposed portions of the copper conductors 102, 104 is avoided provided the etch rate on the horizontal surfaces rival the deposition rate of atoms from the target 18. This condition is realized by an appropriate adjustment of the voltage of the D.C. sputter voltage source 24 (to adjust the deposition rate) and of the power level of the VHF source power generator 36. The power level of the HF (or LF) bias power generator 38 is adjusted to achieve the desired degree of etch selectivity for horizontal surfaces. The result is that the barrier layer 130 is not formed over the horizontal floor of the via 120, so that the process sequence skips the state illustrated in FIG. 6C.

The next step (FIG. 6E) is to deposit a thick copper layer to form the next interconnect layer 200 and vertical conductors extending through the via 120 between the lower interconnect conductors 102, 104 and the upper interconnect layer 200, as shown in FIG. 6E.

The sequence illustrated in FIGS. 8A through 8C shows one advantage of the re-sputtering step of FIG. 6D. FIG. 8A illustrates one of the small openings 124 prior to deposition of the barrier layer 130. FIG. 8B shows how the barrier layer 130 is formed in cases where the re-sputtering step is not carried out simultaneously with the deposition step. Specifically, the barrier layer 130 tends to accumulate with greater thickness near the top edge of the small opening 124 and with smaller thickness near the bottom of the opening 124. The re-sputtering step removes the excess material from the top edge and removes the material from the floor of the small opening and re-deposits it on the vertical side wall, so that the thickness distribution along the vertical side wall becomes more uniform, as indicated in FIG. 8C. The problem is that the thicker accumulation of barrier material near the top edge of the small opening 124 may lead to pinch-off, so that the re-sputter step could have no beneficial effect, the device would fail.

This risk is avoided in the preferred method in which the re-sputter step (FIG. 6D) and the deposition step (FIG. 6C) are carried out simultaneously. In this preferred mode, the process begins with a newly formed small opening 124 in FIG. 9A and transitions directly to a uniform barrier layer 130 in FIG. 9B. The simultaneous deposition/re-sputtering prevents the deposition process from forming significant non-uniformities in the barrier layer 130. This eliminates the risk of the pinch-off effect illustrated in FIG. 8B.

FIG. 10 is a block diagram illustrating the preferred process. In block 310 of FIG. 10, a deposition D.C. discharge plasma is generated around the target 18 (near the ceiling) to deposit atoms from the target onto the wafer. In block 312, a re-sputter capacitively coupled RF plasma is generated near the wafer to produce ions to bombard the wafer to re-sputter the deposited atoms from the target 18. In the step of block 314, plasma RF bias power is applied to the wafer. The bias power is sufficient to render the sputtering highly selective for horizontal surfaces. In block 320, the plasma source power applied to the wafer and the D.C. sputter voltage applied to the target are adjusted relative to one another so that the re-sputter rate is at least nearly as great as the sputter deposition rate.

One embodiment of the PVD/re-sputter reactor of the invention is capable of depositing three different type of layers by operating in three different modes, specifically, in mode (A), a highly conformal layer with uniform sidewall and horizontal surface coverage, in mode (B), a non-conformal layer with little or no sidewall coverage, and in mode (C), a “punch through” layer with good sidewall coverage and good horizontal surface coverage in a large field, but no coverage of bottom surfaces of high aspect ratio openings. The conformal layer of mode (A), illustrated in FIG. 11A, is obtained by applying a relatively low level of D.C. power to the target (e.g., 5 kW), high VHF source power to the wafer (1 kW at 60 MHz) and a low level of HF bias power to the wafer (about 100 W at 13.56 MHz). The non-conformal layer of mode (B), illustrated in FIG. 11B, is obtained under the same conditions except that the HF bias power level is reduced to zero. The “punch-through” layer of mode (C), illustrated in FIG. 11C, is obtained by increasing the bias power to a high level (500 W at 13.56 MHz). The conformal mode is particularly useful for depositing the copper conductor layer. The non-conformal mode is particularly useful for covering the bottom or floor of a via with a low-resistance metal such as tantalum or titanium. The punch-through mode is the preferred mode for depositing the barrier layer (Ta and TaN) in a via.

In some cases, the plasma density distribution may differ between the three modes described above. In order to maintain more uniform plasma density distribution, the current source controller 45 may cause different levels of D.C. current to flow through the electromagnet coil 43 in different ones of the three modes. In any case, the current level furnished by the current controller 45 is preferably optimized to improve the radial plasma ion density uniformity in the process zone.

Each of the three modes described above with reference to FIGS. 11A, 11B and 11C may be implemented by a process controller 210 shown in FIG. 1 whose outputs control the power level of the target high voltage D.C. supply 24, the power level of the VHF plasma source power generator 36 and the power level of the HF or LF bias power generator 38. The process controller 210 may be controlled by a user through a user interface 212, allowing the user to program the controller to have the reactor of FIG. 1 automatically transition between any of the operating states defined above for the conformal mode, the non-conformal mode and the punch through re-sputter mode. The process controller (or processor) 210 therefore has three states to which the user may set or program the processor 210 to enter into. One state is the conformal deposition mode in which the processor 210 sets the D.C. power level of the supply 24 to a low level, sets the power level of the VHF generator 36 to a high level and the HF/LF bias generator 36 to a low level. The another state is the non-conformal mode in which the processor 210 sets the D.C. power level of the supply 24 to a low level, sets the power level of the VHF generator 36 to a high level and the HF/LF bias generator 38 to zero (or nearly zero) level. The remaining state is the punch through state in which the processor 210 sets the D.C. power level of the supply 24 to a low level, sets the power level of the VHF generator 36 to a high level and the HF/LF bias generator 38 to a high level.

The processor 210 may also govern the electromagnet current source 45, so that in each of the three modes (FIGS. 11A, 11B, 11C), the current level is optimized for a more uniform radial distribution of plasma ion density distribution.

The metal target 18 may assume a shape different from the disk shape illustrated in FIG. 1. For example, as shown in FIG. 12, a modified target 18′ having an annular shape may be employed, leaving a center portion 12 a of the ceiling 12 exposed and an annular portion 12′ blocked by the target 18′. The overlying magnetron 22′ has a corresponding annular configuration. Optionally, VHF plasma source power may be applied to the ceiling center portion 12 a by an additional VHF source power generator 66′ (dashed line in FIG. 12). This may be in addition to or in lieu of the VHF source power generator 36 coupled to the wafer support pedestal 14. However, it is preferred to couple VHF source power to the pedestal 14 rather than to the ceiling 12.

FIG. 13 depicts another option, in which a coil antenna 400 may be placed over the ceiling center portion 12′ and coupled to an RF source power generator 410 through an impedance match circuit 415 to produce an inductively coupled plasma. A louvered shield 420 may cover the ceiling center portion 12 a during metal deposition to avoid metal coverage of the ceiling center portion 12 a, so that the coil antenna 400 will not be shielded from the plasma.

FIG. 14 illustrates how the configuration of the coil antenna 400 and target 18′ may be reversed from that illustrated in FIG. 13. In FIG. 14, the coil antenna 400 is in an annular shape while the disk-shaped target 18 is located at the ceiling center portion 12 a. The reactor of FIG. 14 may be modified as follows: The annular coil antenna 400 may be removed and the peripheral portion 12′ of the ceiling 12 may be conductive and coupled to the VHF plasma source power generator 410 through the impedance match circuit 415, as indicated in dashed line in FIG. 14.

FIG. 15 illustrates another alternative embodiment in which the target 18 is configured in an inverted cup shape. As shown in FIG. 16, the cup shape may be of sufficient height to accommodate an array of magnets 450 along its side to enhance ion distribution.

FIG. 17 is a flow diagram of a process carried out in the PVD/re-sputter chamber of FIG. 1. In the step of block 1710, a preclean process is performed by applying no power to the metal target, introducing an etchant precursor gas such as a fluorine-contain gas or a neutral gas such as argon, applying sufficient VHF plasma source power to the wafer from the VHF generator 36 and applying a small amount of HF bias power to the wafer from the HF generator 38. In the following step of block 1720, the dielectric film (e.g., TaN) of a barrier layer is deposited by introducing nitrogen gas and sputtering the metal target (e.g., tantalum) while re-sputtering by maintaining the VHF-driven argon plasma near the wafer and applying bias power to the wafer. In the next step (block 1730), the metal film of the barrier layer is deposited by stopping the supply of nitrogen gas, so that pure metal is deposited while the VHF-driven argon plasma performs re-sputtering. Thus, three successive processes may be performed in the PVD re-sputter reactor of FIG. 1 without removing the wafer from the reactor.

Use of the shield 20 in the reactor of FIG. 1 with the VHF source power supply 36 can be problematic. This is because process control can be lost due to uncontrolled plasma generation on the outside of the shield. This can occur because the RF return path for plasma near the wafer periphery is along the inside (wafer side) surface of the shield 20 and up to the top of the shield 20, and then down the inside surface of the chamber side wall 10 (traveling the entire height of the side wall 10) and along the bottom of the chamber body to the wafer support pedestal 14. At a VHF frequency such as 60 MHz, this RF return path exceeds more than one wavelength, so that along the path there will be several standing wave peaks of the VHF source power at which plasma is likely to be generated. To the extent such a peak falls on the outside of the shield 20, plasma will be generated on the outside of the shield 20, outside of the wafer processing zone enclosed by the shield 20. Such uncontrolled plasma generation will divert RF source power from the wafer process, causing a loss of process control.

This problem is solved by providing the RF connection bridge or conductive tab 60 referred to above with reference to FIG. 2. The RF connection bridge provides an extremely short RF return path between the bottom edge of the shield 20 and the coaxial outer conductor 51 of the wafer support pedestal 14. The RF return path for plasma over the wafer 16 near the bottom of the shield 20 is therefore much shorter than the VHF source power wavelength, so that there are no standing wave peaks along the shield, and therefore no undesirable plasma generation along surface of the shield 20.

FIGS. 18A and 18B illustrate a preferred embodiment of the connection bridge or tab 60 as an elastically deformable cylindrical spring 224 formed of a conductive material (e.g., copper). The cylindrical spring 224 is supported on a conductive table 226 that is connected to the conductive base 51 of the wafer support pedestal 14. As shown in FIG. 18A, when the pedestal 14 is raised to the top position (where it is held during processing), the cylindrical spring 224 is pressed against the bottom edge of the shield 20 so as to be elastically deformed to at least partially conform with the shape of the bottom corner of the shield 20, thereby providing a very low resistance connection to the shield 20. This path extends from the shield 20 through the tab 60 and radially along the base 51 to the coaxial outer conductor 58 of the coaxial RF feed to the pedestal. In FIG. 18B, the pedestal 14 has been withdrawn to a lower position, allowing the cylindrical spring 224 to return to its undeformed cylindrical shape. The cylindrical axis of the spring 224 lies generally parallel with the plane of the wafer 16.

Copper Deposition Using a Single VHF Frequency:

The reactor of FIG. 1 may be used to deposit copper, such as the copper seed layer 114 of FIG. 7, for example. This is accomplished by using copper instead of tantalum as the metal target 18 in the reactor of FIG. 1. However, using the reactor of FIG. 1 for copper deposition involves special problems. One problem involves undesirable formation (during copper deposition) of an overhanging copper protrusion or “neck” on the sidewall of each narrow aspect ratio opening or via (of the type illustrated in FIG. 8B). Specifically, it is more difficult in a copper deposition process to prevent formation of a metal neck or protrusion on the sidewall. The neck is formed from copper sputtered from the top corner edge of the opening onto the opposing surface of the sidewall. Such an overhanging metal formation neck near the top of the opening (FIG. 8B) tends to protrude into the narrow opening or via, eventually enclosing an unacceptable void at the bottom of the opening by closing off the opening. Sputtering is mainly caused by the carrier gas (Argon) ions. Sputtering of the top corner edge of the via or narrow opening is very high relative to other features because the sputtering rate is maximum for ion incidence angles between 30 and 50 degrees. Using a carrier gas such as Argon to promote the formation of kinetic electrons (for VHF plasma generation near the wafer), we have found it difficult to find a process window of power levels of the VHF and HF power sources 36, 38 in which overhang formation or close-off of the opening is avoided. It seems that the HF bias power required to direct a sufficient flux of copper atoms to the bottom of the via opening (for good bottom coverage) has the undesired effect of creating intense re-sputtering of copper from the top corner edge of the narrow opening onto the opposing sidewall surface of the opening to create the problematic copper protrusion on the sidewall (resulting in a deposition profile similar to that illustrated in FIG. 8A). As the sidewall protrusion grows, it can pinch-off the opening, thereby preventing deposition of copper on the floor of the opening. This problem likely is caused by two factors:

-   -   First, such harmful re-sputtering of the copper from the top         corner edge is worse at higher ion energy levels. This is a         serious problem because the HF and LF bias power frequencies of         FIG. 1 produce a wide ion energy distribution having significant         components at very high ion energies (e.g., an ion energy         component corresponding to the peak-to-peak bias voltage).     -   Secondly, selection of a bias power level sufficient to         accelerate a sufficient amount of copper ions to the via floor         necessarily accelerates many more times that number of argon         atoms to the wafer, which sputter away the copper atoms from the         top corner edge of the narrow opening. This happens because         copper and argon have (for present purposes) fairly similar         atomic masses, so that the effect of the bias voltage in         accelerating ions across the plasma sheath at the wafer will be         about the same for both copper and argon. This is problematic         because, due to the limited efficiency of the target 18 of FIG.         1 as a source of copper, there can be one (or more) orders of         magnitude more Argon ions present than Copper ions, so that         Argon sputtering of Copper overwhelms the deposition process. In         some cases, sputtering by Argon ions can also reach the via         floor and compete with copper deposition on the via floor.

We have solved this problem (at least for 65 nm feature size design rules) by applying only VHF source power from the VHF source 36 while applying no HF or LF bias power from the HF/LF source 38. In the preceding description of this specification, a similar mode of operation of the reactor of FIG. 1 was mentioned in which no HF or LF bias power is applied. At nominal VHF power levels (e.g., 100 Watts to 300 Watts), such a mode produces a non-conformal metal deposition (characterized by a relatively thin sidewall coverage) due to lack of re-sputtering without HF or LF bias power.

However, this mode is rendered highly conformal by compensating for the lack of HF or LF bias power by increasing the VHF source power level to a medium level or a high level, until achieving a requisite flux of ions perpendicular to the wafer surface (i.e., parallel to the axis of each high aspect ratio opening) to obtain the desired re-sputtering effects. We have found this achieves the highly conformal effects of the now-missing HF or LF bias power. We have discovered that the lack of HF or LF bias power may be completely compensated by increasing the VHF source power level in this manner. This is believed to be because the VHF source power level from the VHF source 36 of FIG. 1 may be increased to the point at which good via floor coverage from direct copper deposition and good sidewall coverage by copper re-sputtering is obtained. (The re-sputtering effect is discussed at length in previous portions of this specification.) This is possible because increasing the VHF source power level does not significantly increase ion energy but mainly increases the flux of copper ions. This is particularly true if the VHF source power frequency is particularly high, e.g., 60 MHz. Increasing source power at such a VHF frequency does not appreciably increase ion energy at the wafer surface because the frequency is too high for ions to follow the oscillations. Instead, the principal portion (or all) of the source power is consumed in generating kinetic electrons from the carrier gas (which are sufficiently light to follow the source power oscillations). This principal portion is preferably greater than at least 50%. This principal portion may be around 80% at a source power frequency of 60 MHz. As a result, the undesirable effects of HF or LF bias power are avoided while conformal copper deposition is achieved.

The increase in VHF power required to achieve the foregoing effect may be significant. The nominal power range of VHF power at which deposition is non-conformal lies in a range of about 50-300 Watts, depending upon chamber design and other process parameters (pressure, gas composition, gas flow rate, etc.). The desired degree of deposition conformality may be achieved in this novel single frequency conformal mode by increasing the VHF power to a medium level, between approximately 300 to 800 Watts. In some cases, it may be necessary to increase the VHF power to a high level (800-1500 Watts) before realizing a desired degree of conformality. The increase in VHF power required to carry out this single frequency conformal mode is readily ascertained empirically by measuring deposition conformality at increasing VHF power levels.

In summary, a new copper deposition mode of the reactor of FIG. 1 is one in which the metal target 18 is copper, and the only RF power source employed is the VHF plasma source power supply 36. No plasma bias power is applied (i.e., RF power having a sufficiently low frequency to be followed by ions such as copper). In this single frequency mode, the VHF source power level is increased to reach a high degree of conformality in the deposited copper layer. Specifically, the VHF source power is increased until a sufficient flux of copper ions at the wafer is achieved to provide good copper coverage on the floors of high aspect ratio openings with sufficient re-sputtering to provide adequate sidewall copper coverage. As in the other modes disclosed previously in this specification, the High Voltage D.C. supply 24 and the magnetron 22 need only provide a medium amount of power, and the copper plasma generated near the target 18 may or may not be self-ionizing.

Dual Frequency Copper Deposition Modes:

While the single frequency copper deposition mode described immediately above is effective at feature sizes as small as 65 nm, it is felt that copper deposition at smaller feature sizes (e.g., 45 nm, 35 nm) are best accomplished using an HF or LF bias in combination with VHF source power. As discussed above, the VHF power may be increased to increase the flux of copper ions without a proportionate increase in ion energy. This advantage is enhanced by increasing the VHF source power frequency from 60 MHz to 81 MHz. With such an elevated frequency, the principal portion of the source power dedicated to kinetic electron generation (to generate plasma ions) is as high as 90% or 95%, almost none of it increasing ion energy. Thus, the VHF source power level may be increased to an even greater degree (without a significant harmful increase in ion energy) in order to increase copper ion flux at the wafer to increase copper deposition at the via floor and enhance uniform copper re-sputtering for uniform deposition on the via side walls. This feature reduces the required HF bias power level for a conformal copper deposition. For example, the required HF power level may be reduced from 5 kWatts to 3 kWatts, and in some cases to even lower levels (e.g., 1 k Watt). By reducing the HF bias power level, undesirable copper sputtering by ions of the carrier gas (e.g., argon) is reduced accordingly. It should be noted that the HF bias power is of a sufficiently low frequency so that ions near the plasma sheath readily follow its oscillations so that nearly all of its power is consumed in accelerating ions toward the wafer and thereby increasing ion energy.

In order to regulate or prevent re-sputtering of the copper from the top corner via edge to the via sidewall by ions of the carrier gas (e.g., argon ions), a carrier gas is employed having a much lower atomic mass than copper. Specifically, a light inert gas (helium) is employed as the carrier gas. Although helium has a low atomic mass, it nevertheless suffices nearly as well as argon to produce, under the stimulation of the VHF source power, sufficient kinetic electrons to produce an adequate copper plasma near the wafer surface. If the carrier gas ions are much lighter than the copper ions, then they do not sputter copper atoms from the via top corner edge onto the via sidewall at such a fast rate. This feature therefore achieves enhanced control of the re-sputtering process. In some embodiments employing a higher bias power level, there is an additional problem of Argon ions sputtering deposited copper atoms from the via floor faster than the copper deposition rate. This additional problem is solved by using the lighter (Helium) carrier gas, because the lighter Helium atoms do not produce such a high copper sputter rate. As a result, the re-sputtering of copper from the top corner via edge to the via sidewall is better regulated or controlled. Furthermore, the additional problem (encountered in some cases) of removing copper from the via floor by the more populous carrier gas ions is eliminated or at least reduced. An inert gas such as helium is selected because it introduces no additional chemical reactions or effects and does not chemically react with the deposited copper.

Another problem unique to copper deposition is that the surface of the deposited copper has a relatively high surface energy, and is active (like a fluid) during plasma enhanced deposition, enabling it to reduce its surface energy by accumulating into small globs or balls of copper on the surface. The result is formation of a low-quality copper surface. This problem is solved by adding to the carrier gas a species that combines with copper bonds on the deposited copper surface (on the wafer) to reduce the copper surface energy. The preferred choice for such a surface energy-reducing agent is hydrogen. In the helium plasma, diatomic hydrogen molecules dissociate into monatomic hydrogen, which adheres to open copper atomic bonds in the surface of the deposited copper layer. This surface reaction reduces the copper surface energy, allowing the deposited copper atoms to flow during deposition in thin uniformly flat layers, thereby forming a uniformly smooth high quality copper surface.

This embodiment is also effective for removing copper oxide from the deposited copper, or for preventing formation of copper oxide during processing. In removing copper oxide, the hydrogen reduces the copper oxide, capturing oxygen atoms to form water molecules that are released into the chamber.

The hydrogen gas may form as much as 10% of the process gas. If the reactor is specially constructed to handle pure hydrogen gas, then hydrogen gas may constitute all or nearly all of the process gas.

The bias power window or range within which the copper deposition rate at the 65 nm via floor exceeds the neck growth rate on the sidewall is a “65 nm window”. The bias power window or range within which the copper deposition rate at the 45 nm via floor exceeds the neck growth rate on the sidewall is a “45 nm window”, which is significantly narrower than the 65 nm window because of the greater aspect ratio of the opening. Within each of these windows, the copper floor deposition rate exceeds the sidewall neck growth rate sufficiently to allow copper deposition on the via floor to reach a target thickness (e.g., 50-500 Angstroms) before the sidewall copper neck pinches off the opening. The problem was that these windows either did not exist or were too narrow for practical operation.

One way in which the process window is widened in the present invention is by increasing the VHF source power level, which increases the flux of copper ions without a concomitant increase in sputtering or neck growth rate. Another way of widening the process window is to employ a low-atomic mass carrier gas species (Helium). This tends to decrease the neck growth rate by reducing sputtering. Using a low atomic mass carrier gas also tends to increase the copper deposition rate at the via floor, by preventing the removal of deposited copper from the floor surface by sputtering from the carrier gas.

Another advantage of the VHF source power is that it further widens the process window by at least partially masking the nonuniformity in radial distribution of the HF (or LF) bias power. In some cases, bias power radial distribution is non-uniform across the wafer surface, thereby narrowing the bias power process windows. At least partially masking this nonuniformity by applying VHF source power reduces this effect, thereby preserving a wider process window.

In summary, a highly conformal layer of copper is deposited on the floor and sidewall of high aspect ratio openings in the dual frequency reactor of FIG. 1. For this process, the metal target 18 of FIG. 1 is copper. The process gas is about 80% to 95% helium and about 5% to 10% hydrogen. The bias power is either an LF or HF frequency (or a combination of both) at a preferred level of about 20-40 Watts, or under 100 Watts. The source power is a VHF frequency of about 60 MHz, although superior performance may be achieved using a VHF source power frequency of 81 MHz. Similarly, in all of the embodiments described above in this specification, the VHF source power frequency may be increased to 81 MHz to improve process performance. This may allow the VHF source power level to be further increased in order to enhance process performance without increasing the ion energy.

While the invention has been described in detail with reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention. 

1. A method of performing physical vapor deposition of metal in a vacuum chamber of a plasma reactor, comprising: providing a metal target near a ceiling of said chamber; placing a wafer on a wafer support pedestal facing said target; introducing a carrier gas into said vacuum chamber; maintaining a target-sputtering plasma at said target by applying target power from a target power generator to said target to produce a stream comprising at least one of metal atoms and metal ions flowing from said target toward said wafer support pedestal for vapor deposition; maintaining a wafer-sputtering plasma at a surface of said wafer by capacitively coupling plasma RF source power of a VHF frequency through said wafer to the wafer-sputtering plasma by applying said RF source power from a VHF source power generator to a conductor in said wafer support pedestal; and accelerating metal ions of said wafer-sputtering plasma in a direction normal to a surface of said wafer support pedestal by applying bias power of an HF or LF frequency from a bias power generator to said wafer support pedestal.
 2. The method of claim 1 wherein said applying bias power to said wafer support pedestal comprises coupling RF bias power to said wafer support pedestal at a frequency sufficiently low so that a principal portion of the RF bias power contributes to accelerate ions across a plasma sheath.
 3. The method of claim 1 wherein: the frequency of said RE source power is sufficiently high so that the principal portion of said RF source power generates plasma ions in said wafer-sputtering plasma.
 4. The method of claim 1 wherein said maintaining a wafer-sputtering plasma comprises coupling RF source power to said wafer support pedestal at a frequency sufficiently high to excite kinetic electrons in a plasma.
 5. The method of claim 1 wherein said accelerating the metal ions comprises producing sufficient sputtering of metal from horizontal surfaces onto vertical surfaces of high aspect ratio openings on said wafer to deposit a conformal metal layer.
 6. The method of claim 1 wherein said frequency of said RF source power is a VHF frequency sufficiently high so that the principal portion of the RF source power contributes to excitation of electrons in the wafer sputtering plasma.
 7. The method of claim 6 wherein said VHF frequency is about 60 MHz.
 8. The method of claim 6 wherein said VHF frequency is about 81 MHz.
 9. The method of claim 1 wherein said applying target power to said target comprises applying a D.C. voltage to said target.
 10. The method of claim 7 wherein said maintaining a target-sputtering plasma further comprises providing a magnetic field in the vicinity of said target.
 11. The method of claim 1 wherein the power level of said RF plasma bias power is sufficient to enhance sputter etch selectivity for horizontal surfaces for re-sputtering of metal atoms onto vertical surfaces of high aspect ratio openings.
 12. The method of claim 1 wherein said maintaining a target-sputtering plasma and said maintaining a wafer-sputtering plasma are performed simultaneously.
 13. The method of claim 1 wherein said power level of said RF plasma bias power is sufficient to provide a flux of sputter ions relative to a flux of neutral atoms from said sputter target so that the sputter rate of metal from floor surfaces of high aspect ratio openings is on the order of the rate at which said flux of neutral atoms deposits metal.
 14. The method of claim 1 wherein said carrier gas is an inert gas.
 15. The method of claim 1 wherein said carrier gas comprises helium.
 16. The method of claim 1 wherein the principal portion of said source power is consumed in generating plasma ions. 